The use of hydraulic pumps, such as power steering pumps, is well known in the automotive industry. Conventional hydraulic pumps, such as those used in power steering systems, are positive displacement pumps. Positive displacement pumps, such as gear pumps, have a pumping action that creates a pressure fluctuation in the pump discharge flow. Any variations in this pump discharge flow are converted to pressure pulsations when they encounter circuit resistance. This conversion is referred to in the art as pressure ripple.
The pressure pulsations transmitted through the fluid can cause resonating of the system components downstream of the pump, which is known in the art as fluid borne noise. This pressure fluctuation can also excite structure in the pumping circuit causing them to vibrate and generate additional objectionable noise.
Typical pumps generate the majority of their noise energy at the fundamental or pumping frequency (shaft speed x the number of pumping chambers). Typical pumps also can generate appreciable noise energy in multiples of harmonics of the fundamental frequency. The sound level of the noise energy generated by typical pumps generally decreases at higher frequencies, but even fourth and fifth harmonics can have enough energy to cause noise.
Additionally, the configuration of these prior pumps is such that they require the use of hoses for fluid transfer. The hoses are typically located between the outlet of the pump and the steering gear. By adding a pressure pulse attenuator as a separate part downstream of the pump, the size of the power steering gear pack and the cost of the power steering system are increased.
It would therefore be desirable to provide a pump for use in a power steering system that is able to reduce the amount of fluid borne noise and is also compact.